Jesus gives us authority, but he never intended that we should operate under authority alone. We need to be clothed with authority and power. They are not the same thing.

…

We are meant to experientially, practically, live daily being filled with the Spirit. And with the Spirit comes power. That is an important distinction.

Authority is about Position.
Power is about Practice. 
Authority comes from our positional union with Christ. 
Power comes from our practical union with the Spirit. 

Ian Miller, Fasting: Discovering Intimacy With God